Compound polarization beam splitters

ABSTRACT

A compound polarization beam splitter ( 33 ) for use with a reflective, polarization-modulating, imaging device ( 10 ), e.g., a LCoS device, is provided. The compound PBS has: (a) an input prism ( 20 ); (b) an output prism ( 30 ), and (c) a polarizer ( 13 ), which is located between the two prisms ( 20,30 ) and which may be a wire grid polarizer ( 13   a ) or a multi-layer reflective polarizer ( 13   b ). Polarized illumination light ( 11 ) enters the input prism ( 20 ) through a first surface ( 21 ) and undergoes total internal reflection at a second surface ( 22 ) before being reflected from the polarizer ( 13 ) and polarization-modulated at the imaging device ( 10 ). The polarizer&#39;stilt angle (β) is less than 45°, which reduces astigmatism and the required back working distance of the system&#39;s projection lens ( 74 ).

CROSS REFERENCE TO RELATED PROVISIONAL APPLICATION

[0001] This application claims the benefit under 35 USC §119(e) of U.S.Provisional Application No. 60/361,190, filed Feb. 28, 2002, thecontents of which are incorporated herein in their entirety.

FIELD OF THE INVENTION

[0002] This invention relates to optical assemblies (optical units) forthe effective polarization separation of light. The assemblies can beused with, for example, reflective liquid crystal on silicon devices(LCoS devices).

[0003] More specifically, the invention relates to polarizationseparation devices known as polarization beam splitters (also referredto in the art as “polarized beam splitters,” “polarizing beamsplitters,” or simply “PBSs”) and, in particular, to polarization beamsplitters for use in image projection systems which employ one or morereflective, polarization-modulating, imaging devices.

BACKGROUND OF THE INVENTION A. Image Projection Systems

[0004] Image projection systems are used to form an image of an object,such as a display panel, on a viewing screen. Such systems can be of thefront projection or rear projection type, depending on whether theviewer and the object are on the same side of the screen (frontprojection) or on opposite sides of the screen (rear projection).

[0005]FIG. 1 shows in simplified form the basic components of an imageprojection system 77 for use with a microdisplay imaging device (alsoknown in the art as a “digital light valve” or a “pixelized imagingdevice”). In this figure, 70 is an illunination system, which comprisesa light source 71 and illumination optics 72 which transfer some of thelight from the light source towards the screen, 73 is the imagingdevice, and 74 is a projection lens which forms an enlarged image of theimaging device on viewing screen 75.

[0006] For ease of presentation, FIG. 1 shows the components of thesystem in a linear arrangement. For a reflective imaging device of thetype with which the present invention is concerned, the illuminationsystem will be arranged so that light from that system reflects off ofthe imaging device, i.e., the light impinges on the front of the imagingdevice as opposed to the back of the device as shown in FIG. 1. Also, asshown in FIGS. 2 and 3, for a reflective imaging device which operatesby modulating (changing) the polarization of portions of theillumination light (referred to herein as a “reflective,polarization-modulating, imaging device”), a polarization beam splitter(PBS) will be located in front of the imaging device and will receiveillumination light 11, e.g., S-polarized light, from the illuminationsystem and will provide imaging light 12, e.g., P-polarized light, tothe projection lens.

[0007] For front projection systems, the viewer will be on the left sideof screen 75 in FIG. 1, while for rear projection systems, the viewerwill be on the right side of the screen. For rear projection systemshoused in a cabinet, one or more mirrors are often used between theprojection lens and the screen to fold the optical path and thus reducethe system's overall size.

[0008] Image projection systems preferably employ a single projectionlens which forms an image of: (1) a single imaging device whichproduces, either sequentially or simultaneously, the red, green, andblue components of the final image; or (2) three imaging devices, onefor red light, a second for green light, and a third for blue light.Rather than using one or three imaging devices, some image projectionsystems have used two or up to six imagers. Also, for certainapplications, e.g., large image rear projection systems, multipleprojection lenses are used, with each lens and its associated imagingdevice(s) producing a portion of the overall image.

B. Polarization Beam Splitters

[0009]FIG. 2 shows a conventional layout for an image projection systememploying a polarization beam splitter 60 of the MacNeille cube type.See, for example, E. Stupp and M. Brennesholtz, “Reflective polarizertechnology,” Projection Displays, 1999, p. 129-133. As shown in thisfigure, the polarization beam splitter (PBS) consists of two opticallycemented right-angle prisms 61 and 62. The diagonal 63 of the splitterhas a dielectric coating that reflects S-polarized light and transmitsP-polarized light.

[0010] As can be seen in FIG. 2, after reflecting off of the diagonal ofthe MacNeille-type PBS, S-polarized light 14 from the illuminationsystem reaches reflective imaging device 10, e.g., a LCoS device, whereit is polarization modulated. The modulated light 15 is P-polarized andthus passes through diagonal 63 and on to the projection lens to formthe desired image. Non-modulated light (not shown in FIG. 2), which isstill S-polarized, reflects from the diagonal and is returned to theillumination system.

[0011] The main problem with using a MacNeille-type PBS in imageprojection systems is the depolarization of transmitted light that iscaused by skew-ray effects. This is a purely geometrical phenomenon andis described in Miyatake, U.S. Pat. No. 5,327,270, which issued on Jul.5, 1994, and is entitled “Polarizing Beam Splitter Apparatus and LightValve Image Projection System.”

[0012] This depolarized light reduces the contrast of the system. Inaccordance with the Miyatake patent, compensation of the skew-raydepolarization requires an additional quarter-wave plate (i.e., plate 64in FIG. 2), which adds cost, requires precision alignment, and restrictsthe range of operating temperatures. Other disclosures of the use ofcompensating plates in projection systems employing reflective,polarization-modulating, imaging devices can be found in Ootaki, U.S.Pat. No. 5,459,593, Schmidt et al., U.S. Pat. No. 5,576,854, and Bryars,U.S. Pat. No. 5,986,815.

[0013] Another type of PBS is the wire grid polarizer. See, for example,Perkins et al., U.S. Pat. No. 6,122,103, which issued on Sep. 19, 2000,and is entitled “Broadband Wire Grid Polarizer for Visible Spectrum.”This optical component does not suffer from skew-ray depolarization, andalso has a very high polarization extinguish ratio. In addition, thecomponent works over a large temperature range and can withstand a highlight intensity. A wire grid polarizer 13 a can be used with reflective,polarization-modulating, imaging devices, e.g., LCoS devices, inaccordance with the component layouts shown schematically in FIGS. 3Aand 3B. Unfortunately, both of these layouts suffer from opticalproblems.

[0014] The optical problem associated with the layout of FIG. 3A is thatthere is a tilted plano-parallel plate in the imaging optical path. Theplate is the glass substrate (thickness greater than 0.5 mm) thatsupports the wire grid structure. Currently, a technological limitationin the process that creates the wire grid structure makes the use of athinner substrate difficult. A glass substrate 0.5 mm thick, tilted at45 degrees creates astigmatism of −0.135 mm. See Warren J. Smith, ModernOptical Engineering, 2nd edition, McGraw-Hill, Inc., New York, 1990,page 99. The typical depth of focus of a projection lens used with aLCoS device is +/−0.025 mm (for an f-number (F_(No)) of 2.8). Therefore,the layout of FIG. 3A has unacceptable image quality due to astigmatismthat is 2.5-3 times larger than the depth of focus.

[0015] In the layout of FIG. 3B, the light passes through the tiltedglass substrate in the illumination path, where astigmatism is notcritical. In this case, the image quality in the imaging path depends onthe flatness of the wire grid substrate. Acceptable image qualityrequires the surface flatness to be about 1 fringe per inch or better.The best wire grid polarizers available today have a flatness of about 3fringes per inch. There are also two other problems associated with thelayout of FIG. 3B: (1) temperature deformation and (2) wire gridstructure protection.

[0016] Typically, a LCoS projector is assembled and aligned at roomtemperature, but the operational temperature in the area of the LCoS(where the wire grid PBS is located) is 45-55 degrees Celsius. Thiselevated temperature can create deformation of the wire grid substrate,which will degrade the image quality on the screen.

[0017] As to the protection problem, the wire grid structure should beprotected from environmental dust, humidity, mechanical scratches, etc.,which will reduce the polarization properties of the PBS. But any kindof protective window applied in front of the grid structure in theconfiguration of FIG. 3B will essentially reintroduce a plano-parallelplate into the imaging path, which will create astigmatism as discussedabove.

[0018] Another known type of PBS is a multi-layer reflective polarizer.See, for example, Jonza, et al., U.S. Pat. No. 5,965,247, the contentsof which are incorporated herein by reference. See also Private LineReport on Projection Display, Volume 7, No. 11, Jul. 20, 2001, pages6-8.

[0019] Like wire grid polarizers, multi-layer reflective polarizers fallinto the general class of Cartesian polarizers in that the polarizationof the separate beams is referenced to the invariant, generallyorthogonal, principal axes of the polarizer so that, in contrast with aMacNeille-type PBS, the polarization of the separate beams issubstantially independent of the angle of incidence of the beams. SeeBruzzone et al., U.S. Pat. No. 6,486,997, the contents of which areincorporated herein by reference.

[0020]FIG. 3C schematically shows a layout for using a multi-layerreflective polarizer 13 b with a reflective, polarization-modulating,imaging device 10, e.g., a LCoS device. Multi-layer reflectivepolarizers are relatively thick components and, as shown in FIG. 3C, aretilted at an angle of 45 degrees.

[0021] The thickness of this component in combination with thedifference in refractive index between the component and the surroundingglass prisms 51 and 52 creates astigmatism, which degrades the display'simage quality. For example, a multi-layer reflective polarizer can havea thickness and index of refraction of 0.25 millimeters and 1.54,respectively, while the index of refraction of prisms 51 and 52, whencomposed of PBH-56 glass, is approximately 1.85. When tilted at 45°,such an arrangement creates astigmatism of approximately 0.2 mm. Tocorrect this astigmatism, a plano-parallel plate 50 (astigmatismcorrector) having a high index of refraction (e.g., approximately 1.93for PBH-71 glass) can be used next to the multi-layer reflectivepolarizer as shown in FIG. 3C. However, the use of such an astigmatismcorrector significantly increases the cost of the PBS.

SUMMARY OF THE INVENTION

[0022] In view of the foregoing, there exists a need in the art forpolarization beam splitters which have some and preferably all of thefollowing properties:

[0023] (1) the PBS is easy to manufacture and does not require thin orultra-flat substrates;

[0024] (2) the PBS is environmentally protected;

[0025] (3) the PBS is not subject to deformation at elevatedtemperatures; and

[0026] (4) the PBS does not introduce substantial levels of astigmatisminto the image light.

[0027] As an additional fifth property, the optical path through the PBSfor imaging light is preferably short so that the projection lens whichforms the ultimate image can have a shorter back focal length and thus asimpler and less expensive construction.

[0028] To satisfy this need in the art, the invention providespolarization beam splitters for use with reflective,polarization-modulating, imaging panels which have some and preferablyall of the above five features.

[0029] In particular, in accordance with a first aspect, the inventionprovides an image projection system (77) comprising:

[0030] (I) an illumination system (70) which produces polarizedillumination light (11) having a first polarization direction(preferably, S-polarization);

[0031] (II) a reflective imaging device (10) which receives polarizedillumination light (11) and produces modulated reflected light bychanging the polarization direction of selected portions of the receivedlight to a second polarization direction (preferably, P-polarization);

[0032] (III) a projection lens (74); and

[0033] (IV) a prism assembly (33) which comprises an input prism (20),an output prism (30), and a polarizer (13) between the input (20) andoutput (30) prisms,

[0034] wherein:

[0035] (A) the input prism (20) comprises:

[0036] (i) a first surface (21) which receives polarized illuminationlight (11) from the illumination system (70);

[0037] (ii) a second surface (22) which provides polarized illuminationlight (11) to the imaging device (10) and receives modulated reflectedlight from the imaging device (10); and

[0038] (iii) a third surface (23) which faces the output prism (30);

[0039] (B) the output prism (30) comprises:

[0040] (i) a first surface (31) which faces the input prism (20) and isparallel to the third surface (23) of the input prism (20); and

[0041] (ii) a second surface (32) which provides light to the projectionlens (74) to form a projected image; and

[0042] (C) the polarizer (13):

[0043] (i) is between the third surface (23) of the input prism (20) andthe first surface (31) of the output prism (30); and

[0044] (ii) reflects light having the first polarization direction andtransmits light having the second polarization direction;

[0045] wherein the polarized illumination light (11) has an optical pathwhich comprises:

[0046] (i) inward transmission through the first surface (21) of theinput prism (20);

[0047] (ii) total internal reflection at the second surface (22) of theinput prism (20);

[0048] (iii) outward transmission through the third surface (23) of theinput prism (20);

[0049] (iv) reflection from the polarizer (13);

[0050] (v) inward transmission through the third surface (23) of theinput prism (20); and

[0051] (vi) outward transmission through the second surface (22) of theinput prism (20).

[0052] In accordance with a second aspect, the invention provides aprism assembly (33) which comprises an input prism (20), an output prism(30), and a polarizer (13) between the input (20) and output (30)prisms, where:

[0053] (A) the input prism (20) comprises:

[0054] (i) a first surface (21) which is configured and arranged toreceive polarized illumination light (11) from an illumination system(70);

[0055] (ii) a second surface (22) which is configured and arranged toprovide polarized illumination light (11) to an imaging device (10) andto receive modulated reflected light from the imaging device (10); and

[0056] (iii) a third surface (23) which faces the output prism (30);

[0057] (B) the output prism (30) comprises:

[0058] (i) a first surface (31) which faces the input prism (20) and isparallel to the third surface (23) of the input prism (20); and

[0059] (ii) a second surface (32) which is configured and arranged toprovide light to a projection lens (74) to form a projected image; and

[0060] (C) the polarizer (13):

[0061] (i) is between the third surface (23) of the input prism (20) andthe first surface (31) of the output prism (30); and

[0062] (ii) reflects light having a first polarization direction andtransmits light having a second polarization direction;

[0063] wherein the polarized illumination light (11) has an optical pathwhich comprises:

[0064] (i) inward transmission through the first surface (21) of theinput prism (20);

[0065] (ii) total internal reflection at the second surface (22) of theinput prism (20);

[0066] (iii) outward transmission through the third surface (23) of theinput prism (20);

[0067] (iv) reflection from the polarizer (13);

[0068] (v) inward transmission through the third surface (23) of theinput prism (20); and

[0069] (vi) outward transmission through the second surface (22) of theinput prism (20).

[0070] In accordance with a third aspect, the invention provides amethod for producing an image using a polarizer (13) which reflectslight of a first polarization (preferably, S-polarization) and transmitslight of a second polarization (preferably P-polarization), said methodcomprising in order:

[0071] (1) providing polarized illumination light (11) having a firstpolarization direction (preferably, S-polarization);

[0072] (2) introducing the polarized illumination light into a prism(20) having a plurality of surfaces (21,22,23);

[0073] (3) changing the direction of the polarized illumination lightthrough total internal reflection at one of the prism's surfaces (22);

[0074] (4) reflecting the polarized illumination light from thepolarizer (13);

[0075] (5) modulating the polarization of the polarized illuminationlight at a reflective imaging device (10) by changing the polarizationof selected portions of that light to the second polarization, saidselected portions comprising the light which forms the image; and

[0076] (6) transmitting the selected portions through the polarizer (13)and to a projection lens (74) to form the image.

[0077] In accordance with a fourth aspect, the invention provides amethod for producing an image using a polarizer (13) which reflectslight of a first polarization (preferably, S-polarization) and transmitslight of a second polarization (P-polarization), said method comprisingin order:

[0078] (1) providing polarized illumination light having the secondpolarization direction (e.g., imaging light 12 propagated in theopposite direction in FIG. 6);

[0079] (2) transmitting the polarized illumination light through thepolarizer (13);

[0080] (3) modulating the polarization of the polarized illuminationlight at a reflective imaging device (10) by changing the polarizationof selected portions of that light to the first polarization, saidselected portions comprising the light which forms the image;

[0081] (4) reflecting the selected portions having the firstpolarization from the polarizer (13) to form image light (e.g.,illumination light 11 propagated in the opposite direction in FIG. 6);

[0082] (5) introducing the image light into a prism (20) having aplurality of surfaces (21,22,23);

[0083] (6) changing the direction of the image light through totalinternal reflection at one of the prism's surfaces (22); and

[0084] (7) transmitting the image light to a projection lens (74) toform the image.

[0085] In accordance with each of these aspects of the invention, thepolarizer is preferably either a wire grid polarizer (13 a) or amulti-layer reflective polarizer (13 b).

[0086] The reference numbers used in the above summaries of the variousaspects of the invention are only for the convenience of the reader andare not intended to and should not be interpreted as limiting the scopeof the invention. More generally, it is to be understood that both theforegoing general description and the following detailed description aremerely exemplary of the invention, and are intended to provide anoverview or framework for understanding the nature and character of theinvention.

[0087] Additional features and advantages of the invention are set forthin the detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein. The accompanyingdrawings are included to provide a further understanding of theinvention, and are incorporated in and constitute a part of thisspecification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0088]FIG. 1 is a schematic diagram showing the basic components of animage projection system employing a microdisplay imaging device.

[0089]FIG. 2 is a schematic drawing of an optical layout for an imageprojection system employing a reflective, polarization-modulating,imaging device and a conventional PBS of the MacNeille cube type.

[0090]FIG. 3A is a schematic drawing of a first optical layout for animage projection system employing a reflective, polarization-modulating,imaging device and a wire grid PBS.

[0091]FIG. 3B is a schematic drawing of a second optical layout for animage projection system employing a reflective, polarization-modulating,imaging device and a wire grid PBS.

[0092]FIG. 3C is a schematic drawing of an optical layout for an imageprojection system employing a reflective, polarization-modulating,imaging device and a multi-layer reflective polarizer.

[0093]FIG. 4 is a schematic drawing of a compound polarization beamsplitter (PBS) assembly of the present invention.

[0094]FIG. 5A is a schematic drawing showing a wire grid polarizer at afirst position in the compound PBS assembly of FIG. 4.

[0095]FIG. 5B is a schematic drawing showing a wire grid polarizer at asecond position in the compound PBS assembly of FIG. 4.

[0096]FIG. 6 is a schematic drawing in which light rays have been tracedthrough the compound PBS assembly of FIG. 4.

[0097]FIG. 7 is a schematic drawing in which light rays have been tracedthrough a compound PBS assembly of the invention which includes anadditional fold in the illumination path beyond those of FIG. 4.

[0098]FIG. 8 is a schematic drawing illustrating the calculation ofβ_(min).

[0099]FIG. 9 is a schematic drawing illustrating the calculation ofβ_(max).

[0100] In the above drawings, like reference numbers designate like orcorresponding parts throughout the several views. The elements to whichthe reference numbers generally correspond are set forth in Table 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0101] As discussed above, the present invention relates to a prismassembly which utilizes the excellent polarization properties ofCartesian polarizers, including wire grid polarizers and multi-layerreflective polarizers, and can be used with, for example, a LCoS-basedprojection system with the following advantages:

[0102] (1) no special requirements are imposed on the wire gridsubstrate, specifically, no thickness restrictions and no need forultra-flatness;

[0103] (2) complete environmental protection is provided for the wiregrid structure;

[0104] (3) there is no potential for deformation of the polarizer atelevated temperatures;

[0105] (4) in the case of multi-layer reflective polarizers, the systemhas significantly reduced astigmatism thus reducing or eliminating theneed for an astigmatism corrector; and

[0106] (5) the imaging optical path inside of the prism assembly hasreduced length compared to conventional PBSs.

[0107] In broadest terms, the invention is a compound prism consistingof two prisms 20 and 30, e.g., two glass prisms, and a polarizer 13,e.g., a wire grid polarizer or a multi-layer reflective polarizer, asshown in FIG. 4.

[0108] Beginning with the case of a wire grid polarizer, as shown inFIG. 5A, wire grid polarizer 13 a can be optically cemented to the longside 31 (i.e., first surface) of output prism 30 with the grid structurefacing away from prism 30. Output prism 30 is then positioned so thatthere is a small air gap 40 between surface 23 (i.e., the third surface)of input prism 20 and the wire grid structure. This could beaccomplished using a thin spacer between the wire grid and surface 23 toguarantee the desired clearance.

[0109] The air gap between surface 23 and the wire grid structure ispreferably less that 100 microns, which is small enough that theastigmatism it causes does not degrade the image quality. Mostpreferably, the air gap is less than 50 microns. The spacer that createsthis air gap can be a double-sided adhesive film or a layer of materialdeposited in vacuum or any other type of mechanical layer to provide auniform thickness for this gap. Glass beads suspended in an adhesive canalso be used as the spacer. The spacer can be continuous along all edgesof the wire grid polarizer to provide environmental insulation(isolation) of the air gap. Alternatively, this environmental insulation(isolation) can be provided by paint or a mechanical film applied aroundthe air gap on the outside surfaces of the compound prism.

[0110] Raytracing through the compound prism is shown in FIG. 6.Polarized light 11 (S-polarization) from the illumination system entersprism assembly 33 through surface 21 (the first surface) of input prism20, experiences total internal reflection (TIR) from surface 22 (thesecond surface) of that prism, and then reaches the polarizer. The wiregrid structure reflects S polarization towards the LCoS (or otherpolarizing, reflective, pixelized imaging device) as shown. Lightreflected from the “off” pixels of the LCoS has the same polarizationand goes backwards into the illumination system. Light reflected fromthe “on” pixels changes polarization upon reflection and passes throughthe polarizer into the projection lens. The same light paths andpolarization changes occur for the multi-layer reflective polarizerembodiments of the invention discussed below.

[0111] To minimize scattering and/or reflection, the non-opticalsurfaces of the input and/or output prisms can be tilted and/or movedoutward from their positions shown in FIG. 6. For example, the left handface of output prism 30 can be rotated in a clockwise direction in FIG.6 and moved outward by extending surface 31 so as to minimize thescattering and/or reflection of imaging light passing through thisprism.

[0112] With reference to FIGS. 8 and 9, angle β of prism 20 should bechosen to be between β_(min) and β_(max) which can be determined asfollows:

[0113] (1) Angle β_(min) should provide total internal reflection fromsurface 22 of prism 20 for all rays within a given aperture. FIG. 8shows the marginal ray that has the smallest angle of incidence onsurface 22 of prism 20. If that ray undergoes total internal reflectionit means that all rays within aperture ±γ will have TIR. From FIG. 8 itcan be found that:

β=0.5•(γ+α)

[0114] where:

γ=sin⁻¹(1/(2•n ₁ •F _(No))),

α=sin⁻¹(1/n ₁),

[0115] n₁ is the index of refraction of prism 20, and

[0116] F_(No) is the f-number of the projection lens.

[0117] Accordingly, if β≧0.5•(γ+α), then all rays within aperture ±γwill undergo total internal reflection at surface 22.

[0118] (2) Angle β_(max) is chosen to prevent total internal reflectionfrom surface 23 of prism 20 for all rays within a given aperture. Themarginal ray, which should be taken into consideration, is shown in FIG.9. If angle (β+γ) is less than the angle of TIR, then all rays withinthe given aperture will pass through surface 23 and will interact withthe wire grid structure of the polarizer. Accordingly, to avoid totalinternal reflection at surface 23 for all rays within aperture ±γ, thefollowing relationship should be satisfied:

β≦α−γ.

[0119] The following numerical example illustrates the calculation ofβ_(min) and β_(max) for a representative prism material and projectionlens f-number:

[0120] (1) material of prism: glass SF2, n=1.65222, angle of TIR=37.25°;

[0121] (2) aperture in air of ±10° which corresponds to γ=±6.03° inglass;

[0122] (3) the smallest value for angle β_(min) when angle a is equal tothe TIR angle is:

β_(min)=0.5×(6.03+37.25)=21.64°

[0123] (4) the largest value for angle β_(max) when the angle (β+γ) isequal to the TIR angle is:

β_(max)=37.25−6.02=31.22°

[0124] From a practical point of view it is better to have angle βsmaller because it leads to lower levels of astigmatism when amulti-layer reflective polarizer having a low index of refractioncompared to that of prisms 20 and 30 is used (see below).

[0125] In the above calculations, the aperture angle of ±10° in air isbased on the typical f-number (2.8) for optical systems employing LCoSdevices. Other aperture angles can, of course, be used for LCoS systems.Similarly, the same or different aperture angles can be used for othersystems employing polarizing, reflective, pixelized imaging devicesother than LCoS devices.

[0126] It is important to understand that surface 21 of the prism 20(see FIG. 4) is the mirror reflection of surface 23 from surface 22. Inthe illumination optical path, prism 20 works as a plano-parallel plateand does not create any distortion of the illumination beam. It is alsoimportant to understand that surfaces 32 and 22 are parallel to eachother so that there is no beam distortion in the imaging light path.These considerations also apply to the multi-layer reflective polarizerembodiments of the invention discussed below.

[0127] Prism 20 is preferably made of a low birefringence material tomaintain the polarization contrast of the system. Special glasses (e.g.,SF57, PBH56) with low photo-elastic constants can be used or a lessexpensive glass (such as SF2) can be used in combination with anannealing process to reduce internal stresses. Prism 30 can also be madeof special glass or an annealed glass, but can be made of other, lessexpensive, glasses if desired since the polarization state of the lightwhich passes through this prism is not important. Again, theseconsiderations also apply to the multi-layer reflective polarizerembodiments of the invention discussed below.

[0128] A prism assembly having a wire grid polarizer positioned as inFIG. 5A was prepared and tested and found to exhibit a contrast level ofapproximately 200:1. To achieve higher contrast levels, phasecontrol/anti-reflection coatings can be used on surface 23 to compensatefor the depolarization of light and associated phase shift produced bythis tilted glass/air interface. The use of such coatings in connectionwith the Philips-type prisms used with LCoS imagers has previously beendescribed. See, for example, Yamamoto et al., U.S. Pat. No. 5,594,591.See also Keens, U.S. Pat. No. 4,948,228 and the Essential Macleodsoftware program available from the Thin Film Center, Tucson, Ariz.

[0129] The contrast can also be improved by employing phase controlcoatings on TIR surface 22. Again, these coatings can be of the typepreviously disclosed for use with Philips-type prisms. Table 2 setsforth the refractive indices and thickness of a suitable phase controlcoating for use with PBH-56 glass (the substrate) and a β angle of 21°.Similar coatings can be used with other glasses and prism angles.

[0130] In addition to these coatings, enhanced contrast can be achievedby moving wire gird polarizer 13 a to surface 23 of prism 20, as shownin FIG. 5B. The wire grid structure in this case will face air gap 40,which is now adjacent to prism 30, rather than prism 20 as in FIG. 5A.The same approaches discussed above in connection with FIG. 5A can beused to produce air gap 40 for the FIG. 5B configuration and toenvironmentally isolate that gap once formed.

[0131] By using an index matching optical cement between the substrateof wire grid polarizer 13 a and prism 20, the interface between thepolarizer and prism 20 can be made invisible for the embodiment of FIG.5B. In this way, the depolarization that occurs when polarized lightpasses through a tilted interface is avoided. Even residual stress inthe wire grid substrate is not critical because this substrate is thinand birefringence induced by this residual stress is not significant. Itshould be noted that when the interface between the polarizer and prismis made invisible, the considerations regarding TIR at this interface,i.e., the considerations leading to the calculation of β_(max), nolonger apply and thus β only needs to be greater than or equal to0.5•(γ+α). However, as discussed above, β is preferably kept as small aspossible, e.g., just above β_(min).

[0132] Although the foregoing discussion has been in terms of a wiregrid polarizer, it should be noted that the compound prism structure ofthe invention can be used with other types of polarizers. In particular,the compound prism structure can be used with polarizing birefringencefilms such as those manufactured by the 3M Company. Examples ofpolarizing beam splitters using such films can be found in U.S. Pat.No.6,486,997, U.S. Patent Publication No. 2003 0016334, PCT PatentPublication No. WO 02/102,087, U.S. patent application Ser. No.09/878,559, and U.S. patent application Ser. No. 10/159,694, thecontents of each of which is hereby incorporated herein by reference.

[0133] When such a multi-layer reflective polarizer is used, it replacesthe wire grid and its substrate. More specifically, the film is mountedbetween prism 20 and prism 30 of FIG. 4. For this embodiment, air gap 40shown in FIG. 5 is eliminated and thus the film (or optical cement usedto mount the film) touches both surface 23 of prism 20 and surface 31prism 30. Because the air gap is eliminated, the locations of theillumination system and the projection lens shown in FIG. 6 can bereadily interchanged for this embodiment (see the fourth general aspectof the invention set forth in the Summary of the Invention). Asdiscussed above, the illumination system is preferably designed toproduce S-polarized light and thus for both the wire grid and thepolarizing film embodiments, the grid or film, as the case may be, isoriented so that it reflects S-polarization from the illuminationsystem, i.e., it is oriented to produce the raytracing of FIG. 6 forS-polarized illumination light 11.

[0134] As with the wire grid polarizer embodiments, an optical cement isused to mount the multi-layer reflective polarizer to prisms 20 and 30.Where there is a significant difference between the refractive index ofthe prism glass and the refractive index of the polymer film(s) makingup the multi-layer reflective polarizer, the diagonal surfaces of prism20 and prism 30 should have matching anti-reflection (AR) coatings tominimize Fresnel reflections.

[0135] As with the wire grid polarizer embodiments, TIR surface 22preferably includes a phase control coating to compensate for the phaseshift which occurs when polarized light undergoes total internalreflection. As with the FIG. 5B wire grid embodiment of the invention,only the β_(min) limitation applies to the multi-layer reflectivepolarizer embodiments since the interface between the multi-layerreflective polarizer and prism 20 is preferably made opticallyinvisible.

[0136] The value of having β as small as possible to minimizeastigmatism effects introduced by a multi-layer reflective polarizer isillustrated by the following numerical example:

[0137] Material for prisms 20 and 30—PBH-56 (n_(d)=1.8414);

[0138] Multi-layer reflective polarizer—3M Cartesian polarizing film(n_(d)=1.545, thickness 0.37 mm);

[0139] LCoS imager dimensions—10.55×18.76 mm;

[0140] Effective f-number of projection lens—2.0.

[0141] Table 3 shows the axial astigmatism produced by a tiltedplane-parallel plate of index 1.545 in a surrounding media of index1.8414. The last row of the table shows the minimum thickness along theimaging light path for a given angle of the prism diagonal, and the lastcolumn gives reference data for a conventional PBS with a 45° diagonal.

[0142] As can be seen in this table, smaller values of the diagonaltilt, i.e., smaller values of β, result in significantly smaller levelsof axial astigmatism. Although the minimum thickness increases somewhatat the smaller β values, it is still well below the thickness requiredwhen a conventional 45° PBS is used.

[0143] The contrast achieved by some polarizers, including multi-layerreflective polarizers, depends on the angle at which incident lightimpinges on the polarizer. If the angle of incidence becomes too small,contrast can drop below acceptable levels, e.g., below 1000:1. This dropoff is generally color dependent with, for example, the greatestreduction in contrast occurring for short wavelength light, i.e., bluelight. Accordingly, although small values of β are preferred, β shouldnot be made so small that the contrast of the system becomesunacceptable. Preferably, β should satisfy the following relationship:

β≦γ±θ,

[0144] where γ is as defined above and θ is the minimum angle ofincidence that provides a contrast of 1000:1.

[0145] To compensate for residual astigmatism induced by the tiltedmedia of the compound prism, i.e., the materials comprising thepolarizer and any other materials located between prisms 20 and 30,surface 32 of prism 30 and/or surface 22 of prism 20 can have acylindrical shape.

[0146]FIG. 7 shows an alternate embodiment of the invention in which thecompound prism can have an additional reflective surface 24 (the fourthsurface) to fold the illumination beam. In this case, first surface 21is a portion of second surface 22, as shown in FIG. 7. Alternatively,the additional fold can be arranged in an orthogonal direction to theplane of FIG. 7 for the case where the incoming illumination beam is inthe plane of the imaging device and perpendicular to the imaging beam.

[0147] Although specific embodiments of the invention have beendescribed and illustrated, it is to be understood that a variety ofmodifications which do not depart from the scope and spirit of theinvention will be evident to persons of ordinary skill in the art fromthe foregoing disclosure. TABLE 1 Number Element 10 reflective,polarization-modulating, imaging device 11 light from illuminationsystem 12 light to projection lens 13 Cartesian polarizer 13a wire gridCartesian polarizer 13b multi-layer reflective Cartesian polarizer 14S-polarized light 15 P-polarized light 20 input prism 21 first surfaceof input prism 22 second surface of input prism 23 third surface ofinput prism 24 fourth surface of input prism 30 output prism 31 firstsurface of output prism 32 second surface of output prism 33 prismassembly 40 air gap 50 astigmatism corrector 51 prism 52 prism 60conventional PBS 61 right angle prism 62 right angle prism 63 PBSdiagonal 64 quarter wave plate 65 clean-up polarizer 70 illuminationsystem 71 light source 72 illumination optics 73 microdisplay imagingdevice 74 projection lens 75 viewing screen 77 image projection system

[0148] TABLE 2 Refractive Thickness Layer Index (nanometers) Substrate1.842 1 1.4648 24.49 2 2.3443 19.31 3 1.4648 51.75 4 2.3443 19.65 51.4648 204.14 6 2.3443 27.13 7 1.4648 2.62 8 2.3443 82.86 9 1.4648195.83 10  2.3443 19.24 11  1.4648 108.44 Air 1

[0149] TABLE 3 diagonal tilt (degrees) 22 25 30 45 axial astigmatism(mm) 0.020 0.028 0.049 0.323 minimum thickness (mm) 13 12.6 9.8 15

1-22. (cancelled)
 23. A prism assembly which comprises an input prism,an output prism, and a polarizer between the input and output prisms,wherein: (A) the input prism comprises: (i) a first surface whichreceives polarized illumination light from an illumination system; (ii)a second surface which is configured and arranged to provide polarizedillumination light to an imaging device and to receive modulatedreflected light from the imaging device, wherein the first surface andthe second surface are along a same side of the input prism; and (iii) athird surface which faces the output prism; (B) the output prismcomprises: (i) a first surface which faces the third surface of theinput prism; and (ii) a second surface which is configured and arrangedto provide light to a projection lens to form a projected image; and (C)the polarizer: (i) is between the third surface of the input prism andthe first surface of the output prism; and (ii) reflects light having afirst polarization direction and transmits light having a secondpolarization direction; wherein the polarized illumination light has anoptical path which comprises: (i) inward transmission through the firstsurface of the input prism; (ii) total internal reflection at the secondsurface of the input prism; (iii) outward transmission through the thirdsurface of the input prism; (iv) reflection from the polarizer; (v)inward transmission through the third surface of the input prism; and(vi) outward transmission through the second surface of the input prism.24. The prism assembly of claim 23 wherein the second surfaces of theinput and output prisms are parallel.
 25. The prism assembly of claim 23wherein the polarizer is a Cartesian polarizer.
 26. The prism assemblyof claim 23 wherein the polarizer is a wire grid polarizer.
 27. Theprism assembly of claim 23 wherein the polarizer is a multi-layerreflective polarizer.
 28. The prism assembly of claim 23 wherein thesecond surface of the input prism comprises a coating for compensatingfor phase variations in the polarized illumination light which resultfrom the total internal reflection of that light at the second surface.29. The prism assembly of claim 23 wherein the polarizer is air spacedfrom the third surface of the input prism and the third surfacecomprises a coating for compensating for phase variations in thepolarized illumination light which result from the transmission of thatlight through that surface.
 30. The prism assembly of claim 23 whereinthe input prism further comprises a fourth surface at which thepolarized illumination light undergoes reflection before undergoingtotal internal reflection at the second surface.
 31. A prism assemblywhich comprises an input prism, an output prism, and a wire gridpolarizer between the input and output prisms, wherein: (A) the inputprism comprises: (i) a first surface which receives polarizedillumination light from an illumination system; (ii) a second surfacewhich is configured and arranged to provide polarized illumination lightto an imaging device and to receive modulated reflected light from theimaging device; and (iii) a third surface which faces the output prism;(B) the output prism comprises: (i) a first surface which faces thethird surface of the input prism; and (ii) a second surface which isconfigured and arranged to provide light to a projection lens to form aprojected image; and (C) the wire grid polarizer: (i) is between thethird surface of the input prism and the first surface of the outputprism; and (ii) reflects light having a first polarization direction andtransmits light having a second polarization direction; wherein thepolarized illumination light has an optical path which comprises: (i)inward transmission through the first surface of the input prism; (ii)total internal reflection at the second surface of the input prism;(iii) outward transmission through the third surface of the input prism;(iv) reflection from the wire grid polarizer; (v) inward transmissionthrough the third surface of the input prism; and (vi) outwardtransmission through the second surface of the input prism.
 32. Theprism assembly of claim 31 wherein the second surface of the input prismcomprises a coating for compensating for phase variations in thepolarized illumination light which result from the total internalreflection of that light at the second surface.
 33. The prism assemblyof claim 31 wherein the wire grid polarizer is air spaced from the thirdsurface of the input prism and the third surface comprises a coating forcompensating for phase variations in the polarized illumination lightwhich result from the transmission of that light through that surface.34. The prism assembly of claim 31 wherein the input prism furthercomprises a fourth surface at which the polarized illumination lightundergoes reflection before undergoing total internal reflection at thesecond surface.
 35. The prism assembly of claim 31, wherein the wiregrid polarizer is disposed on the third surface of the input prism. 36.The prism assembly of claim 31, wherein the wire grid polarizer isdisposed on the first surface of the output prism.
 37. A prism assemblywhich comprises an input prism, an output prism, and a polarizer betweenthe input and output prisms, wherein: (A) the input prism comprises: (i)a first surface which receives polarized illumination light from anillumination system; (ii) a second surface which is configured andarranged to provide polarized illumination light to an imaging deviceand to receive modulated reflected light from the imaging device; (iii)a third surface which faces the output prism; and (iv) a phase controlcoating disposed on the second surface to compensate for phasevariations in the polarized illumination light produced at the secondsurface; (B) the output prism comprises: (i) a first surface which facesthe third surface of the input prism; and (ii) a second surface which isconfigured and arranged to provide light to a projection lens to form aprojected image; and (C) the polarizer: (i) is between the third surfaceof the input prism and the first surface of the output prism; and (ii)reflects light having a first polarization direction and transmits lighthaving a second polarization direction; wherein the polarizedillumination light has an optical path which comprises: (i) inwardtransmission through the first surface of the input prism; (ii) totalinternal reflection at the second surface of the input prism; (iii)outward transmission through the third surface of the input prism; (iv)reflection from the polarizer; (v) inward transmission through the thirdsurface of the input prism; and (vi) outward transmission through thesecond surface of the input prism.
 38. The prism assembly of claim 37wherein the polarizer is air spaced from the third surface of the inputprism and the third surface comprises a coating for compensating forphase variations in the polarized illumination light which result fromthe transmission of that light through that surface.
 39. The prismassembly of claim 37 wherein the polarizer is a Cartesian polarizer. 40.The prism assembly of claim 37 wherein the polarizer is a multi-layerreflective polarizer.
 41. An image projection system comprising: (I) anillumination system which produces linearly polarized illumination lighthaving a first polarization direction; (II) a reflective imaging devicewhich receives linearly polarized illumination light and producesmodulated reflected light by changing the linear polarization directionof selected portions of the received light to a second linearpolarization direction; (III) a projection lens; and (IV) a prismassembly which comprises an input prism, an output prism, and apolarizer between the input and output prisms, wherein: (A) the inputprism comprises: (i) a first surface which receives linearly polarizedillumination light from the illumination system; (ii) a second surfacewhich provides linearly polarized illumination light to the imagingdevice and receives modulated reflected light from the imaging device;and (iii) a third surface which faces the output prism; (B) the outputprism comprises: (i) a first surface which faces the third surface ofthe input prism; and (ii) a second surface which provides light to theprojection lens to form a projected image; and (C) the polarizer: (i) isbetween the third surface of the input prism and the first surface ofthe output prism; and (ii) reflects light having the first linearpolarization direction and transmits light having the second linearpolarization direction; wherein the linearly polarized illuminationlight has an optical path which comprises: (i) inward transmissionthrough the first surface of the input prism; (ii) total internalreflection at the second surface of the input prism; (iii) outwardtransmission through the third surface of the input prism; (iv)reflection from the polarizer; (v) inward transmission through the thirdsurface of the input prism; and (vi) outward transmission through thesecond surface of the input prism.
 42. The prism assembly of claim 41wherein the polarizer is a Cartesian polarizer.
 43. The prism assemblyof claim 41 wherein the polarizer is a wire grid polarizer.
 44. Theprism assembly of claim 41 wherein the second surface of the input prismcomprises a coating for compensating for phase variations in thelinearly polarized illumination light which result from the totalinternal reflection of that light at the second surface.